This invention relates to a process for producing coatings of polyvinyl fluoride (PVF) on substrates. More particularly, it relates to such a process involving dipping the substrate into a bath.
PVF is generally not soluble at room temperature in conventional solvents, however, it can be put into solution with so-called latent solvents. A dispersion of PVF powder is suspended in latent solvent and heated to a first temperature at which a gel is formed and then to a higher second temperature at which a solution is formed. Latent solvents and other technology useful in handling PVF are discussed in U.S. Pat. Nos. 2,953,818--Barton (Sept. 27, 1980) and 3,139-470--Prengle et al. (June 30, 1964), both incorporated herein by reference.
U.S. Pat. No. 3,574,071--Covin (Apr. 6, 1970), teaches a method for oxidizing an aluminum substrate and then coating it with a fluorocarbon coating such as of tetrafluoroethylene or a copolymer of tetrafluoroethylene and hexafluoropropylene by dipping the substrate into an aqueous bath of the coating composition, with the bath at a temperature between about 32.degree. and 43.degree. C. and the substrate at temperatures below 100.degree. C. and preferably between about 66.degree. and 93.degree. C. Such temperatures are too low for gelling and coalescing coating of the quite different material PVF.
It is difficult to make uniform coatings of PVF due to its thixotropic behavior in latent solvents. Spraying tends to give irregular thicknesses or wavey surfaces such as in an orange peel configuration. Dipping is generally unsatisfactory because of excessive sagging.